Equity Transactions (Detail) - USD ($) $ / shares in Units, $ in Thousands					3 Months Ended				12 Months Ended			75 Months Ended
	Feb. 24, 2017	Feb. 15, 2017	Feb. 01, 2017	Jun. 08, 2016	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016	Feb. 01, 2020
Equity, Class of Treasury Stock [Line Items]
Common stock authorized to be repurchased (in shares)					3,430,000				3,430,000			3,430,000
Remaining common stock authorized for repurchase (in shares)					200,000				200,000			200,000
Repurchase of common shares (in shares)									2,572,417			3,786,561
Cost of shares repurchased									$ 233,700			$ 391,500
Weighted average price (in $ per share)												$ 103.38
Cash dividends declared (in $ per share)					$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 1.8	$ 1.8	$ 1.35
Cash dividends declared									$ 13,015	$ 16,009	$ 12,451
Dividends payable					$ 3,479				$ 3,479	$ 4,233		$ 3,479
Subsequent event [Member]
Equity, Class of Treasury Stock [Line Items]
Cash dividends declared (in $ per share)		$ 0.45
Net proceeds from stock issuance			$ 207,100
Subsequent event [Member] | Common stock [Member]
Equity, Class of Treasury Stock [Line Items]
Stock issued (in shares)			910,000
Net proceeds from stock issuance			$ 207,100
Subsequent event [Member] | Common stock [Member] | Underwriters option [Member]
Equity, Class of Treasury Stock [Line Items]
Stock issued (in shares)	136,500
Net proceeds from stock issuance	$ 14,300
Subsequent event [Member] | Common stock [Member] | Forecast [Member] | Underwriters option [Member]
Equity, Class of Treasury Stock [Line Items]
Stock issued (in shares)	136,500
Subsequent event [Member] | Convertible preferred stock [Member]
Equity, Class of Treasury Stock [Line Items]
Stock issued (in shares)			1,150,000
Preferred stock dividend rate			7.25%
Preferred stock liquidation preference (in $ per share)	$ 100		$ 100.00
Preferred stock conversion, threshold consecutive trading days			20 days
Subsequent event [Member] | Convertible preferred stock [Member] | Forecast [Member] | Minimum [Member]
Equity, Class of Treasury Stock [Line Items]
Preferred stock, shares issuable upon conversion (in shares)													0.7576
Subsequent event [Member] | Convertible preferred stock [Member] | Forecast [Member] | Maximum [Member]
Equity, Class of Treasury Stock [Line Items]
Preferred stock, shares issuable upon conversion (in shares)													0.9091
Stock Purchase Agreement [Member]
Equity, Class of Treasury Stock [Line Items]
Repurchase of common shares (in shares)									1,727,746
Cost of shares repurchased									$ 161,500
Weighted average price (in $ per share)									$ 93.50
Repurchase Plan, Tender Offer [Member]
Equity, Class of Treasury Stock [Line Items]
Repurchase of common shares (in shares)				556,516
Shares repurchased, percentage of common stock outstanding				6.70%